a) Field of the Invention
The present invention relates to filter elements used to filter the dust and dirt particles contained in vapors such as emission gases from various plants. The present invention also relates to a process of manufacturing such filter elements.
b) Description of Related Art
As is well known, hot gases generated from various plants in steel, utility, ceramic, metallurgical, chemical and other fields contain dust particles, and in order to remove them, filter elements of the type described in Unexamined Published Japanese Patent Application (Kokai) Hei. 5-277321 are used; the filter element it teaches is the combination of filter plates that are made of a nonwoven fabric or felt of a polymeric material shaped into a plate having a corrugated cross section.
To produce filter plates having a corrugated cross section, fibers of polymeric materials such as polyimides, polyphenylene sulfides, polyesters and polypropylenes are carded and needle punched to form a nonwoven fabric having a predetermined fiber density, which is then thermally cured and compressed in a press having a corrugated surface profile, whereby the fabric is shaped into a porous filter plate having a corrugated cross section.
A pair of such filter plates are brought together in such a way that the grooves in the corrugated cross section of one filter plate are placed in contact with those in the corrugated cross section of the other filter plate while the ridges in the corrugated cross section of one filter plate are opposed to those in the corrugated cross section of the other filter plate, thereby forming a filter element having a plurality of channels for the passage of the gas to be filtered.
The gas channels in this filter element are connected to an aspiration source so that the dust-containing gas to be filtered is aspirated through the filter element, whereby the dust particles in the gas are trapped on the outer surfaces of the filter plates while a clean gas is yielded within the gas channels.
Using filter plates having a corrugated cross section, the filter element described above has large enough surface areas to permit larger volumes of dust-containing gases to be treated in an efficient manner. Additionally, the sloping side walls connecting the alternating ridges and grooves in the corrugated cross section of each filter plate serve to resist the negative pressure which aspirates the dust-containing gas, thereby contributing to a higher strength of the filter plates taken as a whole.
A problem with the practice of filtering hot dust-containing gases using the aforementioned filter element shaped from nonwoven fabrics or felts of polymeric materials is that as it is used over time, the flexibility of the filter plates increases so that their corrugated geometry will deteriorate progressively under the negative pressure which aspirates the dust-containing gas and that the force of aspiration sometimes exceeds the tensile force of the filter plates to break the filter element.
In order to dislodge the dust particles that have built up on the surfaces of the filter plates, the filter element is given vibrations at periodic intervals of maintenance or a pulsed jet of air is blown into the filter element, but this also causes the problem of deformation of the corrugated filter plates or breakage of the filter element.
Thus, the conventional filter element which uses filter plates made of a nonwoven fabric or felt of polymeric materials that have been shaped into a plate of a corrugated cross section has suffered from the problems of low strength to the filtration of hot dust-containing gases and low durability to a pulsed jet of air.